Reflective optical element for grazing incidence

Abstract

Proposed is a reflective optical element (101) for grazing incidence of radiation having an operating wavelength from the EUV wavelength range, having a reflection layer (107) on a substrate (103), in which a filter layer (105) is arranged between the reflection layer (107) and the substrate (103), wherein the filter layer (105) has a larger real part of the refractive index in the DUV wavelength range than the reflection layer (107).

Description

Reflective optical element for grazing incidence

[0001] The present invention relates to a reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range, having a reflection layer on a substrate. The invention further relates to an optical system having at least two such reflective optical elements. The present application claims the priority of the German patent application 10 2024 205 149.7, the disclosure of which is incorporated in its entirety into the present application by reference.

[0002] In EUV lithography apparatuses, for the lithography of semiconductor devices, use is made of reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g. wavelengths between approximately 5 nm and 20 nm) such as, for instance, photomasks or mirrors on the basis of multilayer systems for quasi-normal incidence or mirrors having a metallic surface for grazing incidence. One possible type of radiation sources are plasma radiation sources. The best known plasma radiation sources are based on laser-produced plasma (LPP source) or on gas discharge-produced plasma.

[0003] Many of the known EUV radiation sources emit radiation not only in the range of an operating wavelength. In order to reduce the radiation dose on the wafer to be exposed due to radiation which is longer-wave than EUV radiation, which is known as out-of-band or interfering radiation, e.g. deep ultraviolet radiation, visible radiation or infrared radiation, it is known from US 7,773,196 B2 to combine two mirrors with one another, of which a first mirror in the beam direction has a higher reflectance for out-of-band radiation and a second mirror in the beam direction has a lower reflectance for out-of-band radiation. For this purpose, the first mirror can have an EUV-radiation-reflecting multilayer system with a sawtooth structure and / or a specific coating, such that EUV radiation and out-of-band radiation is reflected in different directions. The second mirror may have a specific coating that absorbs out-of-band radiation.

[0004] Reflective optical elements for grazing incidence are based on the effect of total internal reflection. In order to achieve high reflectance in the EUV wavelength range, a reflection layer, which preferably has one or more transition metals, is usually provided on a substrate. Ruthenium, molybdenum, niobium and palladium have proven particularly useful. A disadvantage is that, in particular, reflection layers having one or more of the transition metals mentioned likewise have a high reflectance based on total internal reflection for interfering radiation in the ultraviolet to DUV (deep ultraviolet) wavelength range (e.g. wavelengths between approximately 100 nm and 380 nm).

[0005] It is an object of the present invention to specify a possibility for reducing the component of interfering radiation in the work beam.

[0006] The object is achieved by a reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range, having a reflection layer on a substrate, in which a filter layer is arranged between the reflection layer and the substrate, wherein the filter layer has a larger real part of the refractive index in the DUV wavelength range than the reflection layer.

[0007] It has been found that, despite total internal reflection, radiation penetrates via evanescent waves into the reflection layer and interacts there with the material. Here, the penetration depth of the radiation is wavelength-dependent, wherein DUV radiation penetrates deeper than EUV radiation. Providing a filter layer which has a larger real part of the refractive index in the DUV wavelength range than the reflection layer has the positive effect of reducing the proportion of DUV radiation to EUV radiation in the total radiation reflected with grazing incidence. It is assumed that DUV radiation incident by means of the filter layer and penetrating to such an extent is coupled in below the reflection layer and propagates within the filter layer in a manner comparable to a waveguide and is thus virtually no longer available for reflection.

[0008] On account of the improved EUV to DUV radiation ratio, the proposed reflective optical element for grazing incidence is suitable not only for use in EUV lithography apparatuses, but also for metrological applications, for example for measuring masks or other optical elements.

[0009] The filter layer advantageously has a real part of the refractive index of at least 0.65 at a wavelength in the DUV wavelength range. This has proven to be suitable, in particular in the case of EUV wavelengths in the range from approximately 10 nm to 15 nm and DUV wavelengths in the range from approximately 120 nm to 200 nm, for coupling incident DUV radiation particularly well into the filter layer without excessively reducing the intensity of the reflected EUV radiation.

[0010] It has been found to be advantageous if the reflection layer at the operating wavelength has a real part of the refractive index of less than 0.95 and the reflection layer at the operating wavelength has an imaginary part of the refractive index of less than 0.05. As a result, good reflectance in the case of grazing incidence with the lowest possible absorption at the operating wavelength in the EUV wavelength range can be ensured.

[0011] In a preferred embodiment, the reflection layer has a thickness of between approximately 4 nm and approximately 8 nm and the filter layer has a thickness of between approximately 4 nm and approximately 34 nm in order to enable the most efficient input-coupling of DUV radiation into the filter layer with the least possible losses in the EUV radiation.

[0012] Preferably, the filter layer comprises one or more of the materials from the group made up of carbon, calcium fluoride and magnesium fluoride. The carbon may be amorphous carbon, graphite-type carbon or diamond-type carbon. These filter layer materials have significantly different refractive indices than transition metals both in the DUV wavelength range and in the EUV wavelength range, in particular than ruthenium, molybdenum, niobium and palladium, which are in particular suitable for the total internal reflection of EUV radiation.

[0013] Advantageously, the ratio of reflected intensity of radiation in the DUV wavelength range to reflected intensity at the operating wavelength is at most 0.8 at an angle of incidence in the range from 65° to 85°, preferably 75° to 85°. At these angles of incidence, there is not only a sufficient proportion of total internal reflection, but corresponding reflective optical elements can be installed in optical systems for EUV lithography or for measurement devices without too much of a structural outlay.

[0014] In a particularly preferred embodiment, a further layer is arranged between the filter layer and the substrate. By providing the further layer, it is possible to influence possible undesired output-coupling of the DUV radiation which was coupled into the filter layer. In particular, the influence of the substrate material is reduced and the further layer enables a larger selection of substrate materials without input-coupled DUV radiation being coupled out again to any great extent in such a way that it would nevertheless contribute to the interfering radiation.

[0015] Advantageously, the further layer has the same or a larger real part of the refractive index than the reflection layer in order to be able to enable the input-coupled DUV radiation to propagate as far as possible within the filter layer.

[0016] It is particularly advantageous if the reflection layer and the further layer comprise identical material. This not only increases the propagation distance of the DUV radiation coupled into the filter layer, but also simplifies the production process of the reflective optical element.

[0017] Preferably, the further layer has a thickness of between approximately 25 nm and approximately 60 nm in order to limit as much as possible the influence of the substrate on the total internal reflection of the grazing incident radiation and the input-coupling or output-coupling of the DUV radiation.

[0018] In a preferred embodiment, the reflection layer and / or the filter layer and / or, if applicable, the further layer are of a multilayer design. By virtue of at least one of the layers being embodied in multilayered form, it is possible to influence in particular, specifically the actual refractive index, namely both the real part and the imaginary part at a specific wavelength, of the respective layer in order to be able to further improve the ratio of EUV radiation to DUV radiation in the radiation reflected at the reflective optical element.

[0019] Furthermore, the object is achieved by an optical system having at least two reflective optical elements as described. The great advantage of such an optical system is that, with an appropriate design of the respective reflective optical elements proposed here, the suppression of the DUV radiation in the reflected radiation can be made to be wavelength-dependent and thus DUV radiation can be efficiently suppressed over quite a wide wavelength range for the entire optical system. Such optical systems are well suited, inter alia, as constituent parts of EUV lithography apparatuses and of wafer and / or mask inspection systems.

[0020] Preferably, the at least two reflective optical elements have different filter layers, which differ in material, thickness and / or distance from the substrate and / or interface from the reflection layer to vacuum in order to suppress the DUV radiation in the optical system over a particularly broad band and / or particularly efficiently.

[0021] Advantageously, the optical system has an even number of reflective optical elements proposed here for grazing incidence, wherein one half of these reflective optical elements has a first filter layer and the other half of these reflective optical elements has a second filter layer. Alternatively, the optical system advantageously has at least three of the reflective optical elements proposed here for grazing incidence, of which at least one has a different filter layer than the other reflective optical elements. By also providing reflective optical elements for grazing incidence which are of identical design, it is possible to reduce the production outlay and nevertheless to achieve broadband suppression of the DUV radiation in the optical system.

[0022] The present invention will be explained in greater detail with reference to preferred exemplary embodiments. In this respect:

[0023] schematically shows the construction of a first embodiment of the reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range;

[0024] schematically shows the construction of a second embodiment of the reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range;

[0025] shows the reflectance at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a first reflective optical element for grazing incidence;

[0026] shows the reflectance at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the first reflective optical element for grazing incidence;

[0027] shows the reflectance at a DUV wavelength as a function of the angle of incidence and the thickness of the filter layer for the first reflective optical element at a thickness of the reflection layer;

[0028] shows the reflectance at an EUV wavelength as a function of the angle of incidence and the thickness of the filter layer for the first reflective optical element at a thickness of the reflection layer;

[0029] shows the reflectance at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a second reflective optical element for grazing incidence;

[0030] shows the reflectance at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the second reflective optical element for grazing incidence;

[0031] shows the reflectance at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a third reflective optical element for grazing incidence;

[0032] shows the reflectance at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the third reflective optical element for grazing incidence;

[0033] schematically shows the construction of a first embodiment of an optical system having at least one reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range;

[0034] schematically shows the construction of a second embodiment of an optical system having at least one reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range;

[0035] shows the reflectance at an angle of incidence as a function of the wavelength in the DUV wavelength range for different thicknesses of the reflection layer and of the filter layer; and

[0036] shows the reflectance as a function of the wavelength in the DUV wavelength range for various optical systems having at least one reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range.

[0037] schematically illustrates the construction of a first embodiment of a reflective optical element 101 for grazing incidence of radiation having an operating wavelength from the EUV wavelength range. The reflective optical element 101 has a reflection layer 107 on a substrate 103, wherein a filter layer 105 is arranged between the reflection layer 107 and the substrate 103. The reflection layer 107 and the filter layer 105 differ in that the filter layer 105 has a larger real part of the refractive index in the DUV wavelength range than the reflection layer 107.

[0038] In use with customary EUV radiation sources such as, for example, a plasma radiation source, in addition to radiation in the EUV wavelength range, longer-wave interference radiation, in particular from the DUV wavelength range, is also incident on the reflective optical element. Transition metals, among other things, have proven to be suitable as materials for a reflection layer for reflective optical elements for grazing incidence at wavelengths in the EUV wavelength range. The reflection layer may have a single-layer or multilayer design. Multilayer reflection layers can be optimized in terms of the lifetime, for example, compared with single-layer reflection layers. Multilayer reflection layers may have two, three, four, five or more layers, in which all layers may be composed of different materials or two or more layers composed of identical material may be provided. In particular for operating wavelengths in the EUV wavelengths, ruthenium, molybdenum, niobium and palladium have proven useful as materials which the reflection layer should comprise in order to achieve total internal reflection over the largest possible angle of incidence. With regard to a high reflectance, the reflection layer at the operating wavelength has a real part of the refractive index of less than 0.95 and an imaginary part of the refractive index of less than 0.05.

[0039] Since, in the case of total internal reflection of EUV radiation, DUV radiation is generally also always reflected, which can falsify the measurement values during measurements or can lead to exposure errors in the case of lithography methods, a filter layer is provided in the case of the reflective optical elements proposed here, which filter layer has a larger real part of the refractive index than the reflection layer in the DUV wavelength range. Particularly good results were obtained when the filter layer has a real part of the refractive index of at least 0.65 at a wavelength in the DUV wavelength range. Especially in combination with reflection layers comprising ruthenium, molybdenum, niobium and palladium, good results could be achieved with filter layers comprising one or more of the materials of the group made up of carbon, calcium fluoride and magnesium fluoride.

[0040] By utilizing the fact that the penetration depth of DUV radiation is greater than that of EUV radiation, the DUV radiation incident on the reflective optical element can, due to the filter layer having a higher refractive index than in the reflection layer, be coupled into the filter layer, in which the DUV radiation can propagate laterally and is no longer reflected together with the EUV radiation. This can take place in particular without negatively influencing the reflectance of the EUV radiation by virtue of the filter layer being arranged at a distance from the surface to the surroundings at which, owing to the different penetration depths into material, specifically into the reflection layer, a greater proportion of incident DUV radiation is present than of incident EUV radiation. It has thus been found to be advantageous if the reflection layer has a thickness of between approximately 4 nm and approximately 8 nm and the filter layer has a thickness of between approximately 4 nm and approximately 34 nm.

[0041] The ratio of EUV radiation to DUV radiation in the radiation reflected at the reflective optical element can be improved by means of the output-coupling of DUV radiation. In preferred variants of the reflective optical element, the ratio of reflected intensity in the DUV wavelength range to reflected intensity at the operating wavelength is at a maximum of 0.8 at an angle of incidence in the range from 65° to 85°, preferably 75° to 85°.

[0042] In a further embodiment, a reflective optical element 201 proposed here, as illustrated schematically in, may have, in addition to a reflection layer 207 and a filter layer 205, a further layer 209, which is arranged between the filter layer 205 and the substrate 203. By providing the further layer, it is possible to influence possible undesired output-coupling of the DUV radiation that was coupled into the filter layer. In particular, the influence of the substrate material is reduced and the further layer enables a larger selection of substrate materials without input-coupled DUV radiation being coupled out again to a greater extent in such a way that it would nevertheless contribute to the interfering radiation. For this purpose, it has been found to be advantageous if the further layer has the same or a larger real part of the refractive index than the reflection layer. In terms of production engineering, it is particularly advantageous if the reflection layer and the further layer comprise an identical material. Furthermore, the adhesion to the substrate 203 and the roughness of the reflective optical element 201 can be influenced by means of the further layer 209. Overall, the further layer advantageously has a thickness of between approximately 25 nm and approximately 60 nm.

[0043] It should be noted that not only the reflection layer but also the filter layer and / or, if applicable, the further layer can be of multilayer design. In particular, it is thereby possible to influence the actual complex refractive index in order to further improve the coupling of interfering DUV radiation into the filter layer and thus to remove it from the radiation reflected at the reflective optical element with grazing incidence.

[0044] illustrates the reflectance at an EUV wavelength of 13.5 nm as a function of the thickness of the reflection layer and the thickness of the filter layer for a first reflective optical element. The reflective optical element is one which has, on a quartz glass substrate, a reflection layer composed of ruthenium of variable thickness and a filter layer composed of diamond-type carbon of variable thickness and then a further layer composed of 30 nm of ruthenium. The thickness dependence was examined at a wavelength of 13.5 nm and an angle of incidence of 75° with respect to surface normals. In, the reflectance at a wavelength of 120 nm has been examined analogously. The reflectance is plotted in a height line illustration, wherein, in, the lines of the same reflectance have a distance of approximately 0.0080 to 0.0085, with a value of 0.800 for the line of highest reflectance and a value of 0.675 for the line of lowest reflectance. In, the lines of equal reflectance have a distance of approximately 0.0085 with a value of 0.599 for the line of highest reflectance and a value of 0.333 for the line of lowest reflectance.

[0045] In order to suppress the DUV radiation to a greater extent, it is advantageous in principle if the reflection layer is as thin as possible and the filter layer is as thick as possible. Moreover, the filter layer should not only have a real part of the refractive index in the DUV wavelength range greater than the reflection layer, but also an imaginary part as high as possible in order to also absorb the DUV radiation. By contrast, in order not to impair the reflectance of the EUV radiation too much, the reflection layer should in principle have a minimum thickness in the order of magnitude of the operating wavelength and the filter layer should be as thin as possible. Moreover, the reflection layer should have both the smallest possible real part and imaginary part of the refractive index in the EUV wavelength range.

[0046] Despite requirements, in particular with regard to the thicknesses of the reflection layer and of the filter layer, which appear in part contradictory at first glance, solution spaces can be found in which both sufficient EUV reflectance and DUV suppression can be achieved, as can be established for example on the basis of Figures 3 and 4 together. Thus, for example in the thickness range between approximately 4 nm and approximately 10 nm of the filter layer made of amorphous carbon, the reflectance in the DUV range decreases greatly without the EUV reflectance being influenced to a greater extent.

[0047] Comparing a conventional reflective optical element for grazing incidence of EUV radiation having a monolayer of 30 nm as a reflection layer on a substrate with a reflective optical element proposed here having a reflection layer of ruthenium of a thickness of 5 nm on a filter layer of amorphous carbon of a thickness of 6 nm on, in turn, a further layer of ruthenium having a thickness of 30 nm, at an angle of incidence of 75° in each case, a ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm is obtained of a value of 0.868 in the case of the conventional reflective optical element and a value of 0.773 for the reflective optical element with filter layer. The DUV radiation is thus suppressed more by about 10%. If carbon with a diamond structure is instead used for the filter layer, a value of only 0.439 is even obtained.

[0048] This relationship also applies in the case of a change in the angle of incidence. In Figures 5 and 6, the reflectance at a DUV wavelength and, respectively, EUV wavelength is plotted as a function of the angle of incidence and the thickness of the filter layer for a comparable reflective optical element at a thickness of the reflection layer. The DUV wavelength is 160 nm () and the EUV wavelength is 13.5 nm (). The thickness of the reflection layer of ruthenium is 5 nm, the thickness of the filter layer of diamond-type carbon is 6 nm and the thickness of the further layer, likewise of ruthenium, is 30 nm. The reflectance is plotted as a continuous line. For comparison, the reflectance of a reflective optical element without a filter layer, that is to say with a ruthenium layer of 35 nm thickness, is plotted in dashed lines. In particular towards higher angles of incidence of 70° and higher, the influence of the DUV radiation suppression is significantly stronger than the reflection loss in the EUV wavelength range.

[0049] In Figures 7 and 8, the reflectance at an EUV wavelength of 13.5 nm () and a DUV wavelength of 120 nm () are plotted at an angle of incidence of 75° for a second reflective optical element at a variable thickness of the reflection layer and a variable thickness of the filter layer composed of calcium fluoride, wherein this reflective optical element has a further layer having a thickness of 30 nm. The reflection layer and the further layer are composed of ruthenium as in the first example. The reflectance is plotted in a height line illustration, wherein, in, the lines of the same reflectance have a distance of approximately 0.075 with a value of 0.788 for the line of highest reflectance and a value of 0.413 for the line of lowest reflectance. In, the lines of equal reflectance have a distance of approximately 0.02 with a value of 0.56 for the line of highest reflectance and a value of 0.22 for the line of lowest reflectance. For a reflective optical element having a reflection layer composed of 6 nm of ruthenium, a filter layer of 30 nm of calcium fluoride and a further layer of 30 nm of ruthenium, the ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm is 0.183 at an angle of incidence of 75° in each case.

[0050] As a further example, Figures 9 and 10 show the reflectance at an EUV wavelength of 13.5 nm () and a DUV wavelength of 160 nm (), respectively, at an angle of incidence of 75° as a function of the thickness of the reflection layer and the thickness of the filter layer for a third reflective optical element, which has a filter layer composed of magnesium oxide and a further layer composed of molybdenum having a thickness of 50 nm. The reflection layer is again composed of ruthenium. The reflectance is plotted in a height line illustration, wherein, in, the lines of the same reflectance have a distance of approximately 0.054 with a value of 0.792 for the line of highest reflectance and a value of 0.328 for the line of lowest reflectance. In, the lines of equal reflectance have a distance of approximately 0.026 with a value of 0.695 for the line of highest reflectance and a value of 0.326 for the line of lowest reflectance. For a reflective optical element having a reflection layer composed of 7 nm of ruthenium, a filter layer composed of 14 nm of magnesium oxide and a further layer composed of 50 nm of molybdenum, the ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm is 0.657 at an angle of incidence of 75° in each case.

[0051] schematically illustrates an optical system 1101, which, in the present example, has four reflective optical elements 1103, 1105, 1107, 1109 for the grazing incidence of an operating wavelength in the EUV wavelength range.schematically illustrates an optical system 1201, which, in the present example, has five reflective optical elements 1203, 1205, 1207, 1209, 1211 for the grazing incidence of an operating wavelength in the EUV wavelength range. The radiation incident in the respective optical system 1101, 1201 is symbolized by a wavy arrow. Optical systems having two, three, six, seven or more reflective optical elements for the grazing incidence of an operating wavelength in the EUV wavelength range are also possible. Moreover, any desired number of further optical elements (not illustrated here) can be provided, in particular reflective optical elements for smaller angles of incidence, inter alia for quasi-normal incidence. The proportion of interfering radiation in the DUV wavelength range is further reduced at each reflective optical element for grazing incidence. Such optical systems are well suited, inter alia, as constituent parts of lithography apparatuses and of wafer and / or mask inspection systems.

[0052] In both exemplary embodiments illustrated here in accordance withand, at least two reflective optical elements have different filter layers. This has the advantage that the suppression of the interfering radiation in the DUV wavelength range can be set depending on wavelength or angle of incidence. Thus, if the reflective optical elements are appropriately matched, the DUV radiation can be suppressed more efficiently over a larger wavelength range or for different angles of incidence. The filter layers may differ in particular in terms of material and their thickness. By varying the thickness of the reflection layer, it is also possible to set the distance of the filter layers from the surface to the surroundings differently. In addition, it is also possible to influence the suppression of the interfering radiation via the choice of material for the further layer and the thickness thereof.

[0053] In the example illustrated in, the optical system 1101 has an even number of reflective optical elements 1103, 1105, 1107, 1109 for grazing incidence, wherein one half of these reflective optical elements has a first filter layer and the other half of these reflective optical elements has a second filter layer in order to suppress the DUV radiation penetrating into the optical system particularly strongly over different wavelength or angle of incidence ranges.

[0054] In the example illustrated in, the optical system 1201 has at least three reflective optical elements for grazing incidence, specifically five elements 1203, 1205, 1207, 1209, 1211, of which at least one has a different filter layer than the other reflective optical elements in order to suppress the DUV radiation penetrating into the optical system particularly strongly over different wavelength or angle of incidence ranges.

[0055] In, the reflectance is plotted as a function of the wavelength for three reflective optical elements proposed here and a conventional reflective optical element at an angle of incidence of 75°. The conventional reflective optical element has a monolayer of ruthenium having a thickness of 35 nm (continuous line). The reflective optical elements proposed here have a reflection layer composed of ruthenium having a thickness of 5 nm and a further layer composed of ruthenium having a thickness of 30 nm. For this purpose, they have a filter layer of 1.4 nm of carbon (dotted line), of 3.6 nm of carbon (dashed line) or of 5.6 nm of carbon (dashed-dotted line). These are in each case diamond-type carbons. Up to about 105 nm, the DUV radiation is less reflected the thinner the filter layer is. From about 140 nm, the DUV radiation is less reflected the thicker the filter layer is.

[0056] The resulting reflectance of an optical system having four reflective optical elements at an angle of incidence of 75° is illustrated in. An optical system having four conventional reflective optical elements as used inis illustrated as a reference in a continuous line. In addition, the resulting reflectance for two reflective optical elements having a filter layer of 1.4 nm of carbon and two having a filter layer of 5.6 nm of carbon is plotted in dashed lines and the resulting reflectance for four reflective optical elements having a filter layer of 3.6 nm of carbon is plotted in dotted lines, in each case corresponding to the reflective optical elements as used for. From a wavelength of 110 nm, the reflectance decreases significantly compared to the optical system entirely without filter layers, even if the reflectance increases from approximately 175 nm.

[0057] Comparable results can also be obtained with reflection layers of molybdenum, niobium and palladium. Owing to their improved transmission or reflection and lifetime, the optical elements described here are suitable in particular for use in optical systems for lithography apparatuses or mask or wafer inspection systems

[0058] Reference signs

[0059] 101 Reflective optical element

[0060] 103 Substrate

[0061] 105 Filter layer

[0062] 107 Reflection layer

[0063] 201 Reflective optical element

[0064] 203 Substrate

[0065] 205 Filter layer

[0066] 207 Reflection layer

[0067] 209 Further layer

[0068] 1101 Optical system

[0069] 1103 Reflective optical element

[0070] 1105 Reflective optical element

[0071] 1107 Reflective optical element

[0072] 1109 Reflective optical element

[0073] 1201 Optical system

[0074] 1203 Reflective optical element

[0075] 1205 Reflective optical element

[0076] 1207 Reflective optical element

[0077] 1209 Reflective optical element.

Claims

Reflective optical element for grazing incidence of radiation having an operating wavelength from the EUV wavelength range, having a reflection layer on a substrate, characterized in that a filter layer is arranged between the reflection layer and the substrate, wherein the filter layer has a larger real part of the refractive index in the DUV wavelength range than the reflection layer.Reflective optical element according to Claim 1, characterized in that the filter layer has a real part of the refractive index of at least 0.65 at a wavelength in the DUV wavelength range.Reflective optical element according to Claim 1 or 2, characterized in that the filter layer comprises one or more of the materials of the group made up of carbon, calcium fluoride and magnesium fluoride.Reflective optical element according to any of Claims 1 to 3, characterized in that the reflection layer at the operating wavelength has a real part of the refractive index of less than 0.95 and an imaginary part of the refractive index of less than 0.05.Reflective optical element according to any of Claims 1 to 4, characterized in that the reflection layer has a thickness of between approximately 4 nm and approximately 8 nm and the filter layer has a thickness of between approximately 4 nm and approximately 34 nm.Reflective optical element according to any of Claims 1 to 5, characterized in that the ratio of reflected intensity in the DUV wavelength range to reflected intensity at the operating wavelength is at most 0.8 at an angle of incidence in the range from 65° to 85°, preferably 75° to 85°.Reflective optical element according to any of Claims 1 to 6, characterized in that a further layer is arranged between the filter layer and the substrate.Reflective optical element according to Claim 7, characterized in that the further layer has the same or a larger real part of the refractive index than the reflection layer.Reflective optical element according to Claim 7 or 8, characterized in that the reflection layer and the further layer comprise identical material.Reflective optical element according to any of Claims 7 to 9, characterized in that the further layer has a thickness of between approximately 25 nm and approximately 60 nm.Reflective optical element according to any of Claims 1 to 10, characterized in that the reflection layer and / or the filter layer and / or, if applicable, the further layer are of multilayer design.Optical system having at least two reflective optical elements according to any of Claims 1 to 11.Optical system according to Claim 12, characterized in that the at least two reflective optical elements have different filter layers.Optical system according to Claim 12 or Claim 13, characterized in that it has an even number of reflective optical elements according to any of Claims 1 to 10, wherein one half of these reflective optical elements has a first filter layer and the other half of these reflective optical elements has a second filter layer.Optical system according to Claim 12 or Claim 13, characterized in that it has at least three reflective optical elements according to any of Claims 1 to 10, of which at least one has a different filter layer than the other reflective optical elements.

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